$4000 US for all four of you or *each*? And is that the amount of money you'll have before purchasing your plane tickets or after?
I would recommend getting an open-ended ticket, so that you can come home if you run out of cash.
Or just have really understanding parents ready and able to wire you money in the middle of the night if need be!
Some people have managed to make it on $40 or so per day, but personally my minimum is $50/day for a bed, food, sights and transport to different places. There will be days that you don't spend that much and days that you spend more. It all depends on what you want to do. You could also look into camping, but that's going to significantly increase the amount of stuff you need to bring with you. *shrug*
Sounds like fun though!